1. Field of the Invention
The present invention relates to a semi-monolithic ring cavity for a second harmonic generation of laser frequency which includes normal mirrors and a nonlinear crystal, and more particularly to a semi-monolithic ring cavity for a second harmonic generation of laser frequency which includes a nonlinear crystal, an input mirror, an output mirror separated from the input mirror, a first point, at which a fundamental wave is incident onto the input mirror, a second point, at which the fundamental wave is incident onto the output mirror along with a second harmonic wave, and a third point, at which the fundamental wave reflects totally from a side surface of the nonlinear crystal.
2. Description of the Prior Art
Generally, the efficiency of a second harmonic generation of continuous-wave laser frequency is very low. For this reason, an external power build-up cavity is arranged at the outside of a laser. Known configurations of external cavities for a second harmonic generation of laser frequency are classified into three types as follows:
1) Discrete Open Cavity PA1 2) Monolithic Cavity PA1 3) Semi-monolithic Standing-wave Cavity
A discrete open cavity is illustrated in FIG. 2. As shown in FIG. 2, the discrete open cavity includes three or four mirrors (In the illustrated case, four mirrors M1 to M4) which constitute an external cavity. These mirrors M1 to M4 are separated from a nonlinear crystal 5. A fundamental wave is maintained in the form of a ring in the external cavity.
Since such a discrete open cavity includes three or four mirrors separated from a nonlinear crystal and individually mounted to different optical mounts, its external cavity has a size very larger than those of monolithic ring cavities and semi-monolithic standing-wave cavities. Furthermore, it is difficult to obtain a dynamically stable cavity. Where the TEM.sub.00 mode of the discrete open cavity is locked at the frequency of the fundamental wave, this locking is easily broken due to a variety of surrounding noise. The polarized beam of the second harmonic wave is perpendicular to the fundamental wave upon the matching of the first phases on both incident surface of the nonlinear crystal 5. A beam scattering phenomenon occurs on the mirrors M1 to M4. As a result, the optical loss caused by the reflection is large, thereby degrading the second-harmonic wave transform efficiency.
In FIG. 2, the reference numeral 10 denotes a laser, 11 a reflecting beam of the fundamental wave, 12 a piezo-electric drive element for modulation, 13 a piezo-electric drive element for control, and 14 an isolator. The reference character BP denotes a brewster plate arranged for the compensation for an astigmatism.
This monolithic cavity includes two mirrors constituting an external cavity. These mirrors are integral with a nonlinear crystal adapted to generate a second harmonic wave. The formation of the mirrors is achieved by directly machining the nonlinear crystal and coating a dielectric on the machined portions of the nonlinear crystal. In the external cavity, a fundamental wave is maintained in the form of a ring.
Such a monolithic cavity has no drawback involved in the above-mentioned discrete open cavity. That is, the monolithic cavity is very stable in terms of dynamics because the external cavity thereof is integral with the nonlinear crystal. The monolithic cavity also has a high second-harmonic wave generation (namely, transform) efficiency, as compared to the discrete open cavity. However, such a monolithic cavity is problematic in that it involves a very low frequency response speed and a limited continuous frequency tuning range. This is because although the frequency of the monolithic cavity should be tuned simultaneously with the frequency of the fundamental wave while being locked at the frequency of the fundamental wave, the frequency tuning depends on the control for the temperature of the nonlinear crystal in the monolithic cavity.
This semi-monolithic standing-wave cavity includes two mirrors constituting an external cavity. One of the mirrors is a general laser mirror whereas the other mirror is integral with a nonlinear crystal adapted to generate a second harmonic wave. The latter mirror is formed by directly machining one surface of the nonlinear crystal and coating a dielectric on the machined surface of the nonlinear crystal. In the external cavity, a fundamental wave exists in the form of a standing wave.
One of the mirrors in such a semi-monolithic standing-wave cavity corresponds to one mirror surface of the above-mentioned monolithic cavity which is replaced by a general mirror separated from the nonlinear crystal. This mirror is arranged on a piezo-electric drive element capable of achieving a high-speed frequency tuning. Accordingly, the semi-monolithic standing-wave cavity has a high frequency stability and a continuous frequency tuning characteristic. However, this semi-monolithic standing-wave cavity inevitably involves a frequency disturbance of the fundamental wave laser due to the feedback of the fundamental wave which reflects from the incident surface of the semi-monolithic standing-wave cavity and then returns to the laser. For this reason, the semi-monolithic cavity should use an expensive optical isolator. In this semi-monolithic standing-wave cavity, second harmonic waves are generated which travel in the same direction as the travel direction of the fundamental wave and in the opposite direction to the travel direction of the fundamental wave, respectively. Accordingly, it is required to match the phases of the second harmonic waves travelling in both directions in order to obtain a high second-harmonic wave transform efficiency. Under this condition, a second harmonic wave is generated in the same direction as the travel direction of the fundamental wave. Accordingly, it is also required to use a polarization beam splitter in order to separate the generated second harmonic wave from the fundamental wave.